1. Field of the Invention
This invention relates to a wiring board to which a semiconductor chip is connected in a flip chip manner, and relates to a semiconductor device in which a semiconductor chip is connected to this wiring board in a flip chip manner.
2. Description of Related Art
To realize a size reduction and high-density packaging of a semiconductor device, attention has been paid to a flip chip connection structure in which a semiconductor chip is connected to a solid state device while causing a functional surface of the semiconductor chip on which a functional element is formed to face the solid state device.
FIG. 6 is a diagrammatic sectional view of a semiconductor device that has a flip chip connection structure. This semiconductor device 51 includes a wiring board 52 and a semiconductor chip 53 connected to the wiring board 52 while causing a functional surface 53a on which a functional element 54 is formed to face a bonding surface 52a of the wiring board 52.
A plurality of connection electrodes 55 are formed on the bonding surface 52a of the wiring board 52. A solder resist film 56 that has a thickness smaller than a distance between the bonding surface 52a and the functional surface 53a of the semiconductor chip 53 is formed on the bonding surface 52a of the wiring board 52. The solder resist film 56 has a plurality of openings 56a by which each of the connection electrodes 55 is exposed.
A plurality of electrode pads 57 electrically connected to the functional element 54 are formed on the functional surface 53a of the semiconductor chip 53. The electrode pad 57 is exposed from an opening 59a formed in a surface protecting film 59 with which the functional surface 53a is covered. A projection electrode 58 protruding from the surface of the surface protecting film 59 is formed on each of the electrode pads 57.
The connection electrode 55 formed on the bonding surface 52a of the wiring board 52 and the projection electrode 58 formed on the functional surface 53a of the semiconductor chip 53 are connected together via a bonding material 60 made of a low-melting-point metal that is lower in solidus temperature (melting point) than the electrode pad 57, the connection electrode 55, and the projection electrode 58. A solder ball disposed on the projection electrode 58 of the semiconductor chip 53 is melted when the semiconductor chip 53 and the wiring board 52 are bonded together, and, as a result, the bonding material 60 is formed.
A gap existing between the wiring board 52 and the semiconductor chip 53 is filled with an underfill layer 63.
FIG. 7 is a diagrammatic sectional view for explaining a method for producing the conventional semiconductor device 51.
First, the wiring board 52 is substantially horizontally held while directing the bonding surface 52a thereof upward. The semiconductor chip 53 is then held while a rear surface 53b opposite the functional surface 53a is being adsorbed by a bonding tool 62 that has a built-in heater for heating. The semiconductor chip 53 is caused to face the bonding surface 52a of the wiring board 52 while directing the functional surface 53a downward. Solder balls 61 corresponding to the connection electrodes 55, respectively, are formed on the functional surface 53a of the semiconductor chip 53.
Thereafter, a positional adjustment is performed so that the solder ball 61 of the semiconductor chip 53 can come into contact with the connection electrode 55 of the wiring board 52, whereafter the bonding tool 62 is moved down so that the semiconductor chip 53 is bonded to the wiring board 52. At this time, the semiconductor chip 53 is heated by the bonding tool 62, and this heat melts the solder ball 61. Thereafter, the bonding tool 62 stops heating the semiconductor chip 53, so that the solder balls 61 are formed into the bonding material 60 by which the connection electrode 55 and the projection electrode 58 are electrically connected together.
Further, an unhardened (liquid) underfill material is injected into a gap between the wiring board 52 and the semiconductor chip 53, and a hardening process is performed, so that the gap between the wiring board 52 and the semiconductor chip 53 is filled with the underfill layer 63. As a result, the semiconductor device 51 shown in FIG. 6 is obtained.
This semiconductor device and producing method are disclosed by the following document.
Chua Khoon Lam, and another, “Assembly and Reliability Performance of Flip Chip with No-flow Underfills”, 2003 Electronics Packaging Technology Conference, pp. 336-341.
However, the connection electrodes 55 of the wiring board 52 or the solder balls 61 of the semiconductor chip 53 are not equal in height, and therefore, in order to reliably bond the connection electrode 55 and the projection electrode 58 together, a large load must be applied onto the semiconductor chip 53 when bonded together. Therefore, the melted solder ball 61 spreads in a direction along the bonding surface 52a (the functional surface 53a). As a result, disadvantageously, the connection electrodes 55 adjoining in the in-plane direction of the bonding surface 52a (the projection electrodes 58 adjoining in the in-plane direction of the functional surface 53a) are electrically short-circuited by the bonding material 60, and a short circuit defect occurs.
Occasionally, the underfill layer 63 is formed such that an underfill material, which is unhardened, is applied onto the bonding surface 52a before bonding the semiconductor chip 53 to the wiring board 52, and is hardened after the semiconductor chip 53 is connected to the wiring board 52. In this case, in order to bring the solder ball 61 into contact with the connection electrode 55, the semiconductor chip 53 is pressed against the wiring board 52 by the bonding tool 62 with a greater force than a case in which an unhardened underfill material is absent.
If the semiconductor chip 53 is heated by the bonding tool 62 in this state and, as a result, a molten liquid of the solder ball 61 is generated, this molten liquid will easily spread in the in-plane direction of the bonding surface 52a. Hence, the connection electrodes 55 or the projection electrodes 58 adjoining in the in-plane direction are electrically short-circuited by the bonding material 60 formed by the solidifying of the molten liquid, and a short circuit defect is liable to occur.